Prevent Re-initialization of zsh completion system

  * Resolves issue with gcloud sdk completions not working

src/main/bash/sdkman-init.sh

@@ -178,8 +178,11 @@ export PATH
 # source completion scripts
 if [[ "$sdkman_auto_complete" == 'true' ]]; then
 	if [[ "$zsh_shell" == 'true' ]]; then
-		autoload -Uz compinit
-		compinit
+		# initialize zsh completions (if not already done)
+		if ! (( $+functions[compdef] )) ; then
+			autoload -Uz compinit
+			compinit
+		fi
 		source "${SDKMAN_DIR}/contrib/completion/zsh/sdk"
 		__sdkman_echo_debug "ZSH completion script loaded..."
 	elif [[ "$bash_shell" == 'true' ]]; then